1. Field of the Invention
The present invention relates to a current-excitation light-emitting element. In addition, the present invention relates to a light-emitting device and an electronic device having the light-emitting element.
2. Description of the Related Art
In recent years, research and development have been actively conducted on light-emitting elements using electroluminescence. In a basic structure of such a light-emitting element, a layer containing a light-emitting substance is interposed between a pair of electrodes. By applying voltage to this element, light emission from the light-emitting substance can be obtained.
Light-emitting elements using electroluminescence are broadly classified according to whether the light-emitting substance is an organic compound or an inorganic compound.
In the case where an organic compound is used for the light-emitting substance, by applying voltage to a light-emitting element, electrons and holes are injected from a pair of electrodes into a layer containing a light-emitting substance, so that current flows therethrough. Then, by recombination of these carriers (electrons and holes), the light-emitting substance forms an excited state, and emits light when the excited state returns to a ground state. Because of such a mechanism, such a light-emitting element is called a current-excitation light-emitting element.
In such a manner, the current-excitation light-emitting element using an organic compound as a light-emitting substance can be manufactured to be thin and lightweight and can be driven at low voltage. Therefore, the current-excitation light-emitting element is thought to be suitable for a next-generation flat panel display element. In addition, response speed being extremely fast is also one of the characteristics, and it is also an advantage to realize high image quality with the application to the display.
Moreover, since these light-emitting elements are formed in film forms, planar light emission in a large area can be easily obtained. This implies that a “surface light source” which is difficult to obtain by a point light source typified by incandescent lamps and LEDs or by a line light source typified by fluorescent lights can be formed easily. Therefore, utility value as a surface light source that can be applied to lighting or the like is also high.
However, although these light-emitting elements can provide various emission colors depending on kinds of light-emitting substances, development of a light-emitting element emitting white light is important in consideration of application to a flat panel display or lighting. This is because, with combination of a white light-emitting element and a color filter, not only a full-color display can be realized but also white light is an emission color with the highest demand in lighting.
A plurality of organic compounds which show different emission colors are combined, so that light emission having a broad spectrum or white light emission can be obtained. However, luminous efficiency or lifetime of the light-emitting element as described above is a great problem, which is similar to those in a white light-emitting element.
In order to solve these problems, luminous efficiency or lifetime of the white light-emitting element is improved by applying a specific substance in, for example, Patent Document 1: Japanese Published Patent Application No. 2007-201491. However, the measure is yet inadequate in consideration of practical application.